Glossary · Phase 2 · Math, when you need it
Difference-in-differences
A change's effect measured as the treated group's before-and-after change minus the control group's change over the same time.
Try it now
Double the spread and t halves. Quadruple the months and t doubles. The same gap can be convincing or meaningless depending on how noisy and how plentiful the data is.
Learn it properly
Comparing groups: t-tests · Welch's t-test, pooled vs average rates, and a control group for before-and-after changes. About 40 minutes.
Other terms from the same lesson
- control group: A group that didn't get the change, used to show what would have happened anyway.
- t-test: A test of whether two groups' averages differ by more than chance would explain.
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